N-ary Tree Preorder Traversal

给定一个n元树，返回其节点值的preorder.

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val, List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
    public List<Integer> preorder(Node root) {
        List<Integer> list = new ArrayList<>();
        if(root == null)
            return list;
        list.add(root.val);
        for(Node n : root.children) {
            list.addAll(preorder(n));
        }
        return list;
    }
}